It could be simply that your sliding fork stanchions need some lube, or that the seals are squeaking (same issue). Beyond that it depends on the fork. Many low end forks have steel springs inside, and you could be hearing the coil move within the stanchions.

Start with a bit of stanchion lube, which is available at bike shops, or you could fool around with some silicone grease/wax products if you already have them. If that doesn't cure it, let someone knowledgeable listen, as the exact sound will be meaningful to anyone who knows about suspension forks.